The present invention is directed to a method for error compensation in a communication transmission environment. More particularly, the present invention is directed to a method for providing for channel compensation in a multi-carrier tone environment whereby the method minimizes the number of pilot tones needed to estimate a complex channel coefficient.
Wireless communications have become a popular mode by which users can exchange voice and/or data information. In fact, wireless communications, which initially were thought to be primarily useful for establishing mobile communication systems, have also become a popular option for extending access to subscribers without requiring the outlay of additional resources and labor to put wireline configurations into place. For example, it has been proposed to provide a service referred to as “fixed wireless” as an. alternative to wireline connections into local exchange carriers. The fixed wireless service provider would then be able to provide local service to a fixed location, for instance, a subscriber's home, without having to incur the expense of placing cables or wires to each subscriber in a geographic region.
As wireless communications have become more popular, service providers have explored different options for transmitting and receiving these communication signals. In so doing, designers have taken into account the possible negative impacts of wireless communications such as those arising from multi-path fading. It has been determined that orthogonal frequency division multiplexing (OFDM) is an effective scheme for combating adverse effects of multi-path fading. In OFDM a plurality of tones or subcarrier frequencies are used to carry information via an over-the-air channel. FIG. 1 illustrates how a plurality of carrier tones f1 to fk constitute the carrier signal and that signal can be produced at time intervals with the interval selected to avoid the impact of certain delays arising in the communication path. Modulation of information onto the carriers can be performed by a simple inverse discrete Fourier transform (IDFT) which can be implemented very efficiently as an inverse fast Fourier transform (IFFT). In such an arrangement, a receiver needs a fast Fourier transform (FFT) device in processing the received signal to reverse the modulation operation. The spectrum of the subcarriers in the OFDM environment is permitted to overlay to some degree since the orthogonality relationship between the signals provides the appropriate separation between the carriers.
Both coherent and incoherent modulation schemes can be used in OFDM. Since coherent schemes have better performance, they are used in most OFDM systems. In practice, the transmitted symbols transported on the OFDM signals on the over-the-air channel are disturbed by the physical channel which is said to include the transmitter, the propagation channel, and the receiver itself. The disturbance can be represented or characterized in the form of a multiplicative complex coefficient. In the case that the bandwidth of an OFDM channel is sufficiently narrow, one may assume a model with a complex coefficient is common to all the subcarriers (or tones) across the channel. This complex coefficient has to be estimated and then removed or compensated for.
One well known technique for estimating the physical channel coefficient is to transmit one or more pilot symbols along with information symbols on the carriers. By pilot symbols we mean a known symbol at a particular tone. At the receiver, knowing the symbol that is expected to be received on a given tone, the receiver can estimate the complex coefficient. The receiver can then apply the inverse of this coefficient to the other information symbols, thereby compensating for channel disturbances to the information symbols. Following the compensation process, signals from different receiving branches are combined for diversity gain. Decisions as to the content of the information symbols are made based on the combined signals.
Even using this pilot symbol detection technique, symbol errors may arise because the received pilot signals are contaminated by noise. This means that the detected coefficient estimation is inaccurate to some extent. It is expected that the noise will not have the stable characteristics that the remainder of the channel coefficient may have and in fact may vary with time and frequency. Thus, it would be beneficial if there was some way to reduce the effect of this noise since the inaccuracy it introduces will typically degrade the system performance by 3 dB in terms of the signal to noise ratio (SNR).
A number of solutions have been considered in attempting to overcome this problem. One solution is to introduce additional pilots into the system. By using additional pilots there are further reference points for detecting the complex coefficient and noise terms. The drawback from this technique is that with each pilot used the spectrum efficiency of the spectrum is reduced since the number of information carrying tones is reduced. Simply put, additional pilots require additional channel space. A second option is to boost the power of the pilot tone so that the signal-to-noise ratio of the pilot signal is higher than that of the data signals. This would mean that upon detection of the pilot tone it would presumably be a more accurate detection of the complex coefficient as the impact from noise would be smaller or reduced. This solution has its own cost in that as one boosts the power of the pilot tone there is a higher likelihood that there would be interference with adjacent tones.
It would be beneficial if there was a technique for improving the channel compensation operation to take into account the presence of noise without significantly reducing channel capacity or increasing the probability of interference between carrier tones.